I like the idea of an interceptor registered when parsing starts and fitted in `ParseContext` transparently. This interceptor could be notified of successes but also failures. This would make it possible to implement another feature I would like to add to jparsec, namely the ability to recover from failure errors: With a "hook" inside the ParseContext, the interceptor could modify the context in case of errors for example to skip letters/tokens until some bound is encountered (eg. skip to next semi-colon). See issue #3.